PACING THE POST–LAST GLACIAL MAXIMUM DEMISE OF THE ANIMAS VALLEY GLACIER AND THE SAN JUAN MOUNTAIN ICE CAP, COLORADO

During the last glacial maximum (LGM), a 5000 km² ice cap covered the San Juan Mountains of southwest Colorado. The largest valley glacier draining this ice cap occupied the Animas Valley and flowed 91 km to the south. To characterize the post-LGM demise of the Animas Valley glacier, we employ cosmogenic ¹⁰Be to date the LGM terrace 21 km down-valley of the terminal moraines and a suite of seven glacially polished bedrock samples within the LGM glacial footprint. The ¹⁰Be depth profile within the terrace sediments suggests river incision at 19.4 ± 1.5 ka. As deglaciation began, the ponding of Glacial Lake Durango behind the terminal moraines shut off fluvial sediment supply and caused terrace abandonment. The age of the terrace therefore records the initiation of LGM retreat. Negligible ¹⁰Be inheritance in the terrace profile suggests that glacial erosion of the bedrock valley floor from which sediments were derived erased all cosmogenic inventory. Glacial polish exposure ages monotonically decrease up-valley from 17.1 to 12.3 ka with the single exception of a sample collected from a quartzite rib, yielding an average retreat rate of 15.4 m/yr. This trend and the lack of inherited cosmogenic nuclides in the terrace sediments imply that polish ages accurately record the glacial retreat history. Retreat of the Animas lobe began at a time of regional drying recorded in sediments and shoreline elevations of large lakes in the western U.S. Deglaciation lasted for ~7.2 ka, and was complete by 12.3 ± 1.0 ka. The retreat history followed the pattern of increasing insolation and was perhaps fastest during a time of regional drying coincident with the Bolling/Allerod warm phase (~14.7 – 12.9 ka) recorded in Greenland ice cores.